Emerging porous nanosheets: From fundamental synthesis to promising applications
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TRACT Metal-organic framework (MOF) nanosheets and covalent organic framework (COF) nanosheets as emerging porous materials nanosheets have captured increasing attention owing to their attractive properties originating from the advantages of large lateral size, ultrathin thickness, tailorable physiochemical environment, flexibility and highly accessible active sites on surface, and the applications of them have been explored in a wide range of fields. Although MOF and COF nanosheets own many similar properties, their applications in various fields show significant differences, probably due to their different compositions and bonding modes. Hence, we summarize the recent progress of MOF and COF nanosheets by comparative analysis on their advantages and limitations in synthesis and applications, providing a more profound and full-scale perspective for researchers or beginners to understand this field. Herein, the categories of preparation methods of MOF and COF nanosheets are firstly discussed, including top-down and bottom-up methods. Secondly, the applications of MOF and COF nanosheets for separation, catalysis, sensing and energy storage are summarized. Finally, based on current achievements, we put forward our personal insights into the challenges and outlooks on the synthesis, characterizations, and promising applications for future research of MOF and COF nanosheets.
KEYWORDS metal-organic framework (MOF) nanosheets, covalent organic framework (COF) nanosheets, top-down methods, bottom-up methods, two-dimensional (2D) nanomaterials
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Introduction
The dimension of materials plays an important role in their applications. Two-dimensional (2D) nanomaterials show more fascinating properties and promising applications than the zero-dimensional (0D), one-dimensional (1D) and threedimensional (3D) materials [1–3]. As early as 1986, 2D nanomaterials from transition metal dichalcogenides (TMDs), layered double hydroxides (LDHs), graphene, hexagonal boron nitride (h-BN) nanosheets, transition metal carbides or carbonitrides (MXenes) to black phosphorus (BP) nanosheets, have attracted great interest due to their outstanding physical and chemical properties (Fig. 1) [4–15]. Based on the high mechanical flexibility, electronic properties and optical transparency, 2D nanomaterials are broadly used in flexible and transparent electronic devices [16]. Furthermore, the large
surface area stemming from ultrathin thickness and large lateral size enables 2D nanomaterials for applications including catalysis, supercapacitor, gas separation and sensing [17, 18]. These unique properties and applications prompt researchers to develop more kinds of ultrathin 2D nanomaterials. Recent years have witnessed ever-increasing attention paid to metalorganic frameworks (MOFs) and covalent organic frameworks (COFs) due to their unique advantages, which have potential applications in separation, catalysis, sensor, biomedicine and energy storage [19–25]. Specifically, MOFs are usually formed by the coordination of metal ions or clusters with org
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